Basically, a fan blade assembly includes hub structure from which elongated blades radially extend. The hub structure is mounted upon a drive shaft driven by an electric motor, an automobile engine or the like. The fan blades may be riveted to the hub, homogeneously formed of the hub material or may be otherwise affixed. The hub and blades may be of metal and it is also known to mold the entire fan blade assembly, including the hub and the blades, of synthetic plastic material, and in some applications metal hubs are utilized having synthetic plastic blades affixed thereto.
The bending forces imposed upon a fan blade during operation result from a plurality of factors. The rapid rotation of the fan blade imposes centrifugal and centripetal forces on the blades, a variety of vibrational forces are imposed thereon, torsion forces exist endeavoring to twist the blade about its longitudinal axis, and primary bending forces are imposed on the blade resulting from the reaction forces due to the blade's displacement of air as it rotates. These latter forces are particularly significant as they are directed substantially transverse to the thinnest dimension of the blade.
It has long been recognized that the greatest stresses imposed upon fan blades occur at the blade locations adjacent the hub. At such locations the entire bending forces imposed on the blade must be resisted, as well as forces resulting from the blade rotation and tendency to twist, and the most common manner to prevent failure at the root of the blade adjacent the hub is to use reinforcement at this location, such as by widening the blade, using a high strength material, or increasing the blade thickness. These constructions significantly add to the blade cost.
When molding a fan blade of synthetic plastic material the thickness of the blade may be very accurately controlled, as may the blade configuration, and with molded blades formed of synthetic plastic material the overcoming of fracture and failure problems at the blade root can be achieved by using a high strength synthetic plastic, which is very costly, or the thickness of the blade in the root portion may be increased, which requires a significantly greater amount of blade material. Further, increasing the thickness of synthetic plastic fan blades at the root portion adjacent the hub has limitations in that as the thickness is increased the temperature of the blade material in the region of the thickened portion also increases as the blade flexes during operation. The strength of synthetic plastic materials is adversely affected by an increase in temperature, and while the use of a greater thickness of body material does increase the strength in the root portion, such increased masses also retain the heat and permit the heat to build to a temperature which actually reduces the strength of the blade material and can result in blade failure during extended periods of operation. Thus, while improved strength of synthetic plastic blades may be achieved by using more expensive material, such a solution is often economically impractical, and because of thermal strength loss the increasing of the amount of material at stress points is often self defeating.
The basic object of the invention is to provide a fan blade, preferably of synthetic plastic material, wherein the air flow capacity of the blade may be achieved with a minimum amount of blade material while providing resistance to failure, fracture, and other stress-related problems.
It is a further object of the invention to produce a fan blade wherein the load imposed on the blade is distributed over a greater area than usual, and wherein the stresses within the blade remain substantially constant throughout the blade length.
Another object of the invention is to provide a fan blade having the material thereof distributed in the most effective manner relative to a neutral bending axis as to avoid abrupt changes in the blade section modulus or moment of inertia wherein the stresses will remain substantially constant throughout the blade length.
An additional object of the invention is to provide a fan blade of such configuration as to avoid a concentration of load pressures and stresses and wherein stresses on the blade are substantially constant throughout the blade length and, yet, the thickness of the blade is relatively uniform throughout its length.
In the practice of the invention a fan blade, preferably formed of synthetic plastic material, is molded in such a manner that the blade is of a substantially uniform thickness throughout its length.
The blade includes a longitudinal axis, and at its outer region is of the conventional concave-convex configuration defining an air foil air flow portion which is moving at the greatest velocity and displaces the air to be pumped. The innermost region of the blade, i.e. the root, is affixed to a hub, or is integrally molded thereto, and a transition portion exists between the air flow portion and the root portion. It is the configuration of the blade in the transition portion wherein the invention is centered.
From the intersection of the air flow and transition portions radially inward to the root portion the configuration of the transition portion is such that the blade material is increasingly displaced away from the blade neutral bending axis. As the forces imposed on the blade increase toward the root portion, the increasing distance of the blade material from the neutral bending axis permits the blade material to better withstand the bending forces imposed thereon and the blade material is most effectively utilized.
The configuration of the blade through the transition portion is such that the blade material is judiciously located to avoid abrupt changes in the section modulus, torsion modulus and moment of inertia of the blade and the blade configuration, even though of a substantially uniform thickness, permits the stresses therein to remain fairly constant through the transition portion while the moment of inertia uniformly increases and the load imposed on the blade material is distributed over a greater area than accomplished with previous fan blade configurations. As the superior strength of the blade can be achieved without substantially increasing the amount of blade material required the practice of the invention results in significant economies of manufacture without a sacrifice in efficiency, strength, durability and safety factors.